Methods of measuring residual stresses have been in use for some time, and fall generally into two categories: destructive and non-destructive. Destructive methods required that the specimen was cut-up by hole drilling, electrical discharge machining (EDM), sand blasting, or trepanning. Hole drilling was a common method but had the undesirable feature that the drill worked the surface locally and built-in its own stresses. The EDM method while good was awkward to implement. Non-destructive methods have relied on some change in physical property when the material is stressed, e.g., the velocity of ultrasonic waves is affected. Unfortunately the velocity is affected far more by small variations in materials properties which makes the system useless for stress measurement on anything but pure metals. X-rays have also been used but can only measure crystals near the surface and this is a complex method which can be handled well only in the laboratory while the real requirement is to know the stresses over a greater depth, namely those which would influence the initiation and propagation of cracks.